The purpose of this investigation is to test the hypotheses that a) histamine is an important factor in the regulation of gastric and mesenteric microvascular permeability; histamine acts via H1 or H2 receptors or both; and this action of histamine is modulated by endogenous prostaglandin and is also affected by exogenous nicotine and b) the effect of histamine on permeability is separate from its effect on plasma velocity, vessel diameter and intravascular pressure. An in vivo fluorescence microscopy technique will be used to study vascular permeability and plasma velocity. Fluorescein isothiocyanate conjugated to albumin or dextran will be used as the fluorochrome. Television monitoring and videotaping will be performed using a SIT (silicon intensifier target image tube) TV camera, monitor and videotape recorder. Vascular permeability and plasma velocity changes will be determined by an analog analysis system, and vessel diameter changes by an image-shearing monitor on video-tape playback of each experiment. In separate studies, intravascular pressure changes will be determined by means of servo-nulling system employing micropipettes. Studies to be performed will include histamine dose response studies alone and with histamine receptor antagonists, prostaglandin, indomethacin and nicotine. Results of the studies will be analyzed in terms of the hypotheses proposed, and results in the two microvascular beds compared.